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Flight delays, missed connections, stuffy cabins, narrow seats, screaming children. As if commercial air travel isn’t hard enough, now comes evidence that it could be harmful to your health.

In recent lawsuits and public complaints, dozens of passengers and airline personnel report inflight chemical exposures that triggered serious illnesses. The illnesses are similar to those associated with “Toxicant-induced Lack of Tolerance,” or TILT, a clinical condition that breaks down a person’s natural resistance to chemical compounds.

The Boeing Co. recently settled a suit out of court in Seattle with a former American Airlines flight attendant. She alleged that contaminated cabin air in an MD-82 jetliner caused her to have tremors, memory loss and severe headaches. US Airways pilots and flight attendants have filed a similar suit involving a Boeing 767. They reported headaches, sore throats, eye irritations, dizziness and nausea.

A similar incident in 2011 involving a Lufthansa Airlines jumbo jet came to light in December 2013. The trade publication Aviation Herald said the crew aboard the Airbus A380 detected a “chemical odor of dirty socks” when the plane took off from Frankfurt en route to San Francisco.

One flight attendant reported feeling ill after the incident and has been unable to return to work due to headaches, fatigue, poor concentration and eye irritation, the publication said. Subsequent lab tests on the flight attendant identified exposure to the neurotoxin tricresyl phosphate, an organophosphate used in aviation lubricants, the publication said.

At issue are so-called “fume events” where petroleum smells enter the aircraft cabin. Commercial airliners routinely pump, or “bleed,” compressed air to the cabin from outside. Nearly all airliners use this “bleed-air” ventilation system. The airline industry and government regulators say the system is safe, and has been in use since the 1950s. The industry says the number of reports of illness is minuscule compared to the thousands of people who fly each day.

Critics say bleed-air systems draw a substantial amount of air past the engine, thus raising the risk that petroleum fumes can enter the cabin. Loose engine seals and poor aircraft maintenance raise the chances that air can become contaminated on its way to the cabin.

Air travel is difficult for people with chemical intolerances, but until recently no one suspected that taking a flight might trigger illnesses in otherwise healthy people. Finding solutions is urgent because the sheer volume of passenger traffic may leave hundreds of thousands of people exposed and vulnerable to a new sickness.

On any plane, the people most affected by a lack of fresh air usually have asthma, suffer migraines, or are already chemically intolerant. It’s fortunate that the largest amount of fresh air is sent to the flight deck, where the pilot and co-pilot fly the plane. But even pilots and co-pilots have reported illnesses and, in a few cases, even impaired cognitive function.

Even under normal conditions, an air traveler’s “breathing zone” is frequently violated on flights. Some examples:

Boarding the plane, passengers sometimes enter a hot plane with little fresh air. Some of them get headaches or have difficulty breathing. Crews often turn off the PAC, or air-conditioning system, to save fuel. My advice: Ask the crew to turn it on.

Once seated, passengers encounter unpleasant odors — from perhaps from a cigar smoker seated nearby, those wearing fragrances, or others using nail polish. Again, as a passenger, your breathing zone is violated, and you’re stuck for the duration of the flight.

Waiting for takeoff, passengers sometimes breathe exhaust from other planes lined up ahead of them.

Also, the fragrance from restroom deodorizers can cause problems. I was on a recent flight and the odor was intense. Putting potent deodorizers in tiny spaces can cause a very high concentration of fragrance. Passengers also carry the fragrance back to their seat. Recommendation: Sit in the middle of the plane. That way fewer passengers will drag their restroom fragrance vapor trails past your aisle.

Facts and narratives about airplane illnesses are becoming more widely known. MSNBC.com reporter Jim Gold has written an excellent article about the situation.

Passengers may have few defenses, but scientists are working to minimize exposures aboard airplanes. In his article, Gold describes efforts to develop a biomarker for TCPs, or tricresyl phosphates, one of the suspected contaminants in fume events. One key researcher, Dr. Clement Furlong of the University of Washington, said the goal is to better understand the chemistry of the incidents so refiners can develop less-toxic engine fluids. This might improve the bleed-air system, or at least minimize its potential risks to health.

TCP is the cause of numerous poisonings and is a neurotoxin, in part via organophosphate-induced delayed neuropathy. It has been responsible for many deaths. The most serious incidents occurred in the 1920s when TCP was used to adulterate Jamaica Ginger, and in Morocco in 1959 when cooking oil was adulterated with jet-engine lubricant containing TCP.

TCP’s mechanism of action is similar to other organophosphates in that it can inhibit the enzyme acetylcholinesterase, leading to a buildup of acetylcholine in the synaptic space. This can lead to hyperactivity in cholinergic neurons in the brain, and at neuromuscular junctions in the peripheral nervous system resulting in apoptosis of those cell types. This is the reason for paralysis and other irreversible neurological problems seen in the “Gingerjake” syndromes during Prohibition, when TCP was added to gingerjake moonshine.

Dr. Furlong is working to develop a blood test to prove with a biomarker that someone aboard an aircraft during a fume event was exposed to TCPs. A reliable test would dispel the uncertainty now in the air for air travelers.

TCP is one of many substances capable of initiating TILT. However chlorpyrifos, or Dursban, is one of the most common causes. Why has chlorpyrifos initiated so many illnesses? You may want to review a paper I co-authored in the Archives of Environmental Health about initiators. Note mention of TCP on page 121.

Symptoms and health effects of TILT often accumulate over time. People can gauge their sensitivities using the free QEESI questionnaire. It is a clinical tool to evaluate whether someone has TILT or is acquiring intolerances.

Illustrations depict chemical structures of TCP, left, and chlorpyrifos, or Dursban, which is one of the most commonly reported initiators of TILT.